This invention relates to nonwoven fabrics. More particularly, the present invention relates to nonwoven fabrics composed of thermoplastic resin fibers and to methods for manufacturing such fabrics. The nonwoven fabrics of the present invention are configured in such a way as to be useful in constructing absorbent products such as disposable diapers, adult incontinence pads, and sanitary napkins. The nonwoven fabrics of our invention are especially useful as coverstock and as spacer fabrics in absorbent personal care products.
Disposable diapers, sanitary napkins, and the like are generally composed of an impermeable outer covering, an absorbent layer, and an inner layer that--ideally--permits liquid to flow through it rapidly into the absorbent layer ("rapid strike through") but does not permit or at least does not facilitate re-transmission of liquid from the absorbent layer to the baby or wearer side of said inner layer ("resists rewet"). Said inner layer is referred to as coverstock, topsheet, or, in diaper applications, diaper liner. In addition to liquid transport properties, the coverstock must have sufficient strength to allow for converting it--that is, incorporating into the final product--on a diaper or other machine and for resistance to failure during vigorous movement by the user. On the other hand, while strength is essential, the coverstock should present a soft comfortable feel against the user's skin. The subjective feel--softness and dryness--of diaper liner has become more important with the increased use of diapers by incontinent adults. Currently these somewhat conflicting requirements--for softness coupled with strength--have been met only imperfectly, for the most part by coverstock made from thin low basis weight carded or spunbonded nonwoven fabrics.
Recently some absorbent products have been constructed with a "spacer" layer between the absorbent layer and the thin coverstock layer. The spacer layer can provide several functions including fluid acquisition, distribution including lateral liquid transport or "wicking", and separation. Body fluid is often discharged in gushes. The spacer layer must quickly acquire the flood of liquid and transport it by wicking from the point of initial introduction to many parts of the absorbent layer. Distribution and wicking have become of greater importance with the use of expensive superabsorbent polymers (SAP) as part of the absorbent layer. Full utilization of the absorbent material insures economic use of the SAP and prevents gel blocking. The liquid transport aspects of a spacer or fluid acquistion/distribution layer is described more fully in U.S. Pat. No. 4,673,402 . The spacer layer can also improve diaper dryness by increasing the distance or separation between the thin topsheet and the wet absorbent core. A bulky, porous, compression-resistant nonwoven fabric can be used as the spacer layer to yield superior softness, liquid distribution, and surface dryness.
Many of the advantages promised by use of a spacer layer can be achieved using a conventional diaper design if the thickness or caliper of that diaper coverstock fabric is increased. It has been recognized that many aspects of coverstock performance could be substantially improved if the thickness, or caliper, of the coverstock fabric were increased. Surface dryness can be improved by increasing the separation between the wearer's skin and the absorbent core of the diaper. A thick bulky diaper liner could also provide many of the liquid acquisition, distribution, and wicking functions expected from a spacer layer. Since these functions must be maintained during use of the diaper, it is essential that the thick diaper liner maintain its caliper under some degree of compression loading. Thickness can be increased by increasing the basis weight of the coverstock and/or by decreasing the density thereof (that is, by making the coverstock more lofty). Increased thickness through loft should offer improved softness as well as improved surface dryness.
Many approaches have been suggested for producing thick diaper liner. For example, U.S. Pat. No. 4,041,951 teaches embossing nonwoven topsheet to increase its bulk, and U.S. Pat. No. 4,391,869 discloses limiting the amount of aqueous binder applied in the suction bonding of airlaid nonwoven fabric. More recently, the use of thru-air bonded bicomponent fiber structures have been investigated. One use of the thru-air technique is alluded to in an article entitled "Multi-layer Nonwovens for Coverstock, Medical, and other End Uses" by J. Pirkanen in the November 1987 issue of "Nonwovens World". The reference discloses a multilayer nonwoven fabric having a basis weight of about 30 grams per square meter. U.S. Pat. No. 4,548,856 and U.K. Patent Application GB 2,127,865A disclose thru-air bonding procedures that involve the use of multibelt systems to form patterned nonwoven fabrics.
U.S. Pat. No. 4,652,484 assigned to Kao teaches that improved diaper liner will result from a layered structure wherein the first layer is predominently comprised of 1-3 denier "straight" bicomponent fibers and the second layer is predominently comprised of sterically buckled (three-dimensional crimp) 1.5 to 6 denier bicomponent fibers.
Copending U.S. patent application Ser. No. 07/184,228 discloses diaper liner having properties of thickness, softness, and strength comparable to the Kao products that can be manufactured using flat-crimped (rather than sterically-buckled) bicomponent fibers and that achieves such results at substantially reduced basis weights compared to the basis weights of comparable webs described by the Kao patent.
A major practical problem with high loft nonwoven fabrics used for diaper applications such as coverstock or spacer fabrics is that very large diameter soft rolls are generated upon winding relatively short linear yardage thereof. This tends to make shipping more expensive. The soft roll can easily be damaged. Diaper machine efficiency is compromised since short roll lengths require frequent roll changes during the conversion process.
A solution to the problem of large diameter rolls is to make a condensed nonwoven web that can be bulked into a lofty web just before or during diaper manufacture. This approach is well known in the art of powder bond structures. Powder bonding, however, requires the need for expensive infrared oven systems, powder applicators, and costly polyester powder adhesives. It is difficult, if not impossible, to achieve the superior balance of caliper and softness using bulked powder bond structures that can be obtained with lofty thru-air bonded bicomponent fabrics. The present invention, which provides methods to form compressed webs that can be transformed into soft lofty webs with properties that approach those of a never-compressed bicomponent thru-air bonded structure, is a major advance in the art.
U.S. Pat. No. 4,601,937, assigned to Akzona Incorporated, teaches a way to reversibly densify nonwoven webs consisting of the steps of first heating while under compression followed by cooling under compression. The resulting densified web can then be transformed to a lofty low density web by heating without compression. During the first heating step, a temperature below that which changes the state of fiber aggregation is specified. The examples and the description of the invention suggest that the Akzona disclosure is concerned only with nonwoven fabrics used in clothing and industrial application and having basis weights of 80 g/m.sup.2 through 200 g/m.sup.2.
U.S. Pat. Nos. 3,911,641; 3,927,504; 3,964,232; 3,991,538; and 4,163,353 describe methods for packaging very flexible compressible materials such as are used for building insulation.
U.S. Pat. No. 3,669,788 teaches an approach for making bulky acetate fiber nonwoven webs by the steps of extruding a solution of cellulose acetate to form continuous filaments, agitating the filaments while they are in a mutually adhesive state so they become randomly bonded, collecting the filaments in a flat bonded sheet, and then contacting the sheet with steam at temperature of 95.degree.-180.degree. C. such that the web becomes bulky with a significant increase in loft and softness. The description in this patent is limited to webs made using organic acid esters of cellulose such as cellulose acetate.
British Patent 1,334,735 teaches a method for making bulky products by first adhesively bonding a plurality of spaced filaments of a heat shrinkable fiber to a base nonwoven web and then subjecting the resulting product to sufficient heat to shrink the filaments (i.e. contract them in a longitudinal direction), thus causing the fabric itself to shrink with consequent increase in bulk. The description in this patent is limited to a layered structure wherein the two layers are made of fibers having significantly different heat histories.
We have now discovered two approaches to forming a compressed web that can then be transformed into a lofty web with properties nearly matching those of never-compressed bicomponent thru-air bonded fabrics.
In the first approach a bicomponent-fiber based thru-air bonded web is compressed in a nip, preferably as it exits the thru-air bonding oven. A roll of compressed web thus results. It has now been discovered that re-exposure of this compressed web to the proper choice of temperature will regenerate a lofty web with many properties similar to those seen in the initial thru-air bonded never-compressed web.
In the second approach, a bicomponent-fiber-based thru-air bonded product is formed as a lofty web but is then wound under sufficient tension to compress the lofty structure of the web and to obtain a hard compact roll. When the web from such a roll is removed from the compact roll and exposed to heat, a lofty structure can be regenerated. This lofty structure will show a degree of compression resistance similar to that seen for the initial never-compressed lofty thru-air bonded web.